First and second conventional technologies for an air flow measuring device will be described in reference to FIGS. 4 and 5.
The first conventional technology will be explained below. In an air flow measuring device 401 illustrated in FIG. 4, a sensor portion 403 is disposed in a measured passage 402 (passage through which measured air that is a measuring object flows), and an upstream bent portion 404 is provided at an upstream region (upstream in the measured air) of a sensor arrangement portion (region where the sensor portion 403 is disposed) (see JP-T-2004-519690 corresponding to US2003/0159501A1). In this technology described in JP-T-2004-519690, an upstream throttle α′ that reduces a passage area is provided at a further upstream region of the upstream bent portion 404, and an airflow toward the sensor portion 403 is straightened through this upstream throttle α′. Therefore, in the measured passage 402 of JP-T-2004-519690, the upstream throttle α′, the upstream bent portion 404, and the sensor arrangement portion are arranged in this order along a flow direction of the measured air.
However, according to the technology in JP-T-2004-519690, due to the upstream bent portion 404 between the upstream throttle α′ and the sensor arrangement portion, when air is exfoliated on an inner wall surface 404a of the upstream bent portion 404, a flow of air toward the sensor arrangement portion is disturbed. Accordingly, a defect of an increased fluctuation of output from the sensor portion 403 is caused (increased variation in output characteristics and deteriorated output fluctuation).
The second conventional technology will be explained below. In an air flow measuring device 501 illustrated in FIG. 5, a sensor portion 503 is disposed at a curved portion of a measured passage 502 (see JP-A-2008-197122 corresponding to U.S. Pat. No. 6,332,356B1). In this technology described in JP-A-2008-197122, the measured passage 502 on an upstream side of the sensor portion 503 is provided to gradually reduce a passage area up to the sensor portion 503, and an airflow at a sensor arrangement portion is thereby straightened.
However, according to the technology in JP-A-2008-197122, the reduction of the passage area converges at the sensor arrangement portion, so that passage resistance is increased through the measured passage 502, which changes pulsation characteristics. Furthermore, in the technology of JP-A-2008-197122, because the throttle is only in one direction perpendicular to a sensor opposing direction, a defect of an increased output fluctuation of the sensor portion 503 is caused (increased variation in output characteristics and deteriorated output fluctuation).